Serial numbers for Adobe applications can become a real headache on Macintosh computers. Once you have installed an Adobe program, whether it is a retail copy or a demo, the serial number is stored in a file that automatically enters that information when you attempt to install or update Adobe programs. The result is an error message that says the license for a product has expired. Properly removing the old Adobe serial allows you to enter the new, correct serial number.
Uninstall the new Adobe program if you have installed it before, removing the serial number. This action can be completed by going to Applications > Utilities > Adobe Installers.
Download the Adobe CS3 or CS4 Clean Script from the Adobe website. Run the Clean Script.
Remove the cache file that contains your old serial number by going to Library > Application Support > Adobe > Adobe PCD > cache. Locate the cache.db file. Drag it to the Trash and go to File > Empty Trash. When you reinstall and open your Adobe product, you will be prompted to enter a new serial number.